Various plier-type tools are known, such as bolt cutters, wire strippers, sheet-metal benders, nut crackers, punches, and the like, which have a pair of handles that are displaced toward each other in order to move a pair of grippers, blades, or the like together on the workpiece in question. A mechanical advantage is often gained by forming the blades separately from the handles and providing a linkage between these handles and the blades so as to multiply the force exerted by the user on the handles. Such provision is particularly necessary in devices for cutting bolts, reinforcing rods, and the like where the mechanical resistance of the workpiece may require a force of at least 140 hectobars for penetrating the workpiece with the blades.
The most common type of bolt cutter has a so-called knee linkage for multiplying the force exerted on the arms or handles. In this arrangement each handle has its forward end pivoted only on the forward end of the other handle, and each blade is pivoted on each handle at a location somewhat behind this common pivot. Otherwise the two blades are pivoted on a support plate for swinging either about respective axes or on another common axis.
In such a system it is necessary to provide a relatively complicated arrangement to ensure symmetry of movement of the two blades. Furthermore the mechanical requirements of the structure require that the the two blades and handles be different from each other, thereby increasing cost of production and complicating repair.
Another great disadvantage of the known cutters is that the force-multiplication arrangement operates kinematically so that the pressure exerted by the blades on the workpiece is in inverse proportion to the distance between the outer ends of the handle arms. It has been discovered, however, that the force with which the average workpiece resists penetration of the blades usually increases at a greater rate, so that after a predetermined depth of penetration the force that can be comfortably exerted by the user is insufficient to overcome the material hardness. Only when another, higher limit is passed does penetration commence again, usually simultaneously with a complete cutting through of the workpiece. Of course in reality neither of these characteristics can be represented as a straight-line graph.
Thus it is necessary with the prior-art cutters of the above-described type that the force exerted by the operator increases considerably during a portion of the cutting operation. This increase is usually called for at a time when kinematically it is very inconvenient for the operator, because of position of the handles, to augment the force he is applying. To this end recourse is frequently had to twisting the cutter once the blades have penetrated to an extent corresponding to that penetration beyond which a greatly augmented force will be necessary for further cutting. This twisting causes the parts of the cutter to work loose and quickly dulls the blade or blades, depending on whether two cutting blades are provided or just one cutting blade and an anvil blade.